Charging a 12v car battery...

Discussion in 'General Electronics Chat' started by Grayham, Jun 9, 2010.

  1. Grayham

    Thread Starter Member

    May 18, 2010
    79
    2
    Hi,

    Just a quick question. I want to charge (fully) a car battery (connected or not to the car don't mind).

    I have a 13.7v (typically around 13.9v) power source which I after doing some homework have decided will work.

    My question is how do I connect this to the car battery, do I just do + to + and - to - using (high current 12 or 8 AWG) cable?

    Or do I need to current limit with a high power resistor?
     
  2. Bychon

    Member

    Mar 12, 2010
    469
    41
    Yeah, + goes to + and - goes to -
    but you haven't defined the current your supply can provide. a 12 ga wire will handle 20 amps, all day long, and 8 ga will haul 40 amps, but most people don't have a 20 or 40 amp charger, and when you get to 40 amps, you are in danger of hurting the car battery.
     
  3. VoodooMojo

    Active Member

    Nov 28, 2009
    503
    53
    yes, and depending on the current draw from the battery while using an accessory when the charger is charging the battery could damage the power supply (charger) if it cannot handle the current demand.
     
  4. Grayham

    Thread Starter Member

    May 18, 2010
    79
    2
    My power supply is a 13.7v bench supply rated at 12A (with a 10A fuse I think it was in there).

    I guess what I'm asking is, how much current will the car battery pull from the supply? Enough to blow the fuse?
     
  5. VoodooMojo

    Active Member

    Nov 28, 2009
    503
    53
    That depends on how discharged the battery(s) becomes and what side of the charger the 10amp fuse is at. The AC side or the DC side.
    The lower the battery voltage gets, the more the current will be flowing between the charger and the battery.

    Most automotive and mobile equipment battery chargers have a low voltage cut-out that wont let them begin a charge cycle if the battery(s) being charged are too discharged. This protects the charger from being damaged by the excessive current being demanded.
     
    Last edited: Jun 10, 2010
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    If the battery develops a shorted cell, you will have maximum current flow from the bench supply.

    It is not really a good idea to charge a battery from a constant voltage source. You need the current to be limited until the battery is bulk-charged enough to use voltage controlled charging.

    I suggest that your battery charging current should be limited to 5A. Charging standard lead-acid automotive batteries faster than that can lead to internal heating, and that decreases their service life. If you increase the internal temperature by 30°C, the battery life is cut by 2/3.
     
  7. Grayham

    Thread Starter Member

    May 18, 2010
    79
    2
    The 10A fuse is on the DC side, though the AC board also has a breaker but not sure what it's rated as.

    Is there a device you recommend to current limit between the + from source and the + on battery cable? It's not as easy as using a 4.7ohm wirewound resistor is it?
     
  8. Bychon

    Member

    Mar 12, 2010
    469
    41
    The hard part is that you don't have enough voltage to waste any on a regulator chip and still get the job done when the battery is near full. That pretty much puts you in the resistor game. 13.7 volts can throw amps into a battry that is down to 11 volts, so I'd try the 4.7 ohms at 10 watts and put a 1 amp fuse or circuit breaker in series.
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    If the battery is truly and completely dead (<1v at the terminals) then you'll get nearly 3A current through the 4.7 Ohm resistor, until the fuse blows.

    It'll also take forever to charge it completely, due to the decreased current when the battery is fairly well charged.

    You could use a high-wattage automotive bulb, like a 60W headlamp as a current limiting resistor. The nice thing about incandescent bulbs is that they have very low resistance when they are cold, but high resistance when they are hot. You could use a couple of brake lamp bulbs in parallel, too.
     
  10. Grayham

    Thread Starter Member

    May 18, 2010
    79
    2
    I don't have any of these bulbs, but I do have a heap of high power wirewound resistors (ranging from around 3ohms to 15ohms) all rated at over 50w or 100w.

    These should work the same as a bulb right?
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    No, they won't work the same.

    A fixed resistor will have the same resistance no matter the voltage across it.

    An incandescent lamp changes resistance as it heats up; the hotter it is, the higher the resistance.

    You could go to the auto breakers and get a couple of brake lights still in the sockets for next to nothing. Just snip the wires off.
     
    Last edited: Jun 10, 2010
    Grayham likes this.
  12. Potato Pudding

    Well-Known Member

    Jun 11, 2010
    684
    92
    No the resistors won't work the same as a bulb, but they will work. I would use the 15 ohm power resistor monitor the battery charge with a multimeter, and leave it connected for about 3 days in a protected spot outside. Insulated but low fire risk containment would be adviseable. Don't try to speed charge a car battery and especially NEVER indoors. Trickle charging should always be preferred respecially if you are just trying to make something work.

    A commercial battery charger on sale might also be worth looking for, because you are dealing with enough power in charging a car battery that mistakes can be very serious.
     
    Grayham likes this.
  13. Grayham

    Thread Starter Member

    May 18, 2010
    79
    2
    Ok thanks for your advice. I think I may just buy a proper car battery charger from Auto Barn.
     
  14. rickmartin

    New Member

    Sep 26, 2009
    27
    1
    This is slightly off the topic of the question, but it's worth mentioning. If you are new to battery charging--especially lead acid batteries, please be mindful that if anything goes wrong in your circuit that results in the battery being charged at an excessive current, the battery can heat up internally and explode, creating a dangerous situation if anyone is near it with hot acid and shrapnel flying around. Best to wear goggles, make sure the battery has good ventilation, and try to prevent sparks in the area of the battery--especially after a long charge. Further, defects in a charging circuit (especially the home-brewed variety) can also effectively drop a short across the battery, also resulting in it exploding. I don't mean to preach, but I was using a cheap commercial charger the other day and it overcharged my truck battery, which subsequently exploded. The plastic fill plug cap from the battery was found about 40 feet away. Buying a good charger is the way to go.
     
  15. Grayham

    Thread Starter Member

    May 18, 2010
    79
    2
    Thanks for the heads up :)
     
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